This invention relates in general to optical scanners and in particular to an improved shading correction system for improving the performance of optical scanners.
When an optical target is scanned by an optical scanner, the performance of the scanner is affected by a number of factors. The light source for illuminating the target, such as an elongated fluorescent lamp, may emit light of non-uniform intensity along the length of the lamp. Light reflected by the target is usually scanned by an image sensor for sensing light reflected from different portions of the target. A lens is usually used for projecting light reflected by the target onto the image sensor. The action of the lens is such that light transmitted through the center portion of the lens is of higher intensity than light transmitted through the edges of the lens according to the cosine-fourth-power law, in a phenomenon known as vignetting of lenses.
In order to reduce errors, a conventional technique uses a reference target such as a white mark which is first scanned by an optical scanner. The light intensity signals detected by the image sensor when the white mark is scanned are recorded as shading profile values stored for the optical scanner system. The scanner may be of the type where the image sensor is moved between a number of predetermined positions for scanning different portions of a target. Thus for each particular position of the image sensor for sensing a predetermined portion of the target, a shading profile value is stored for shading correction. When an actual target is then subsequently scanned, the output of the image sensor at each of the predetermined positions is then corrected using the shading profile value for such position of the image sensor to reduce errors caused by non-uniform lighting effects and vignetting of the lens.
While the above shading correction scheme does reduce errors caused by non-uniform lighting and vignetting of lens, the result achievable by such scanning systems are still not entirely satisfactory and further improvements are desirable.